1. Field of the Invention
This invention relates to a ferroelectric liquid crystal composition. More particularly it relates to a ferroelectric liquid crystal composition having a negative dielectric anisotropy comprising an achiral compound having a negative dielectric anisotropy and a light switching element using the same.
2. Description of the Related Art
In recent years, liquid crystal display has come to be broadly employed for display elements, utilizing the specific features thereof, such as light weight, small power consumption, etc. However, most of these display elements utilize TN display mode using liquid crystal materials having a nematic phase; hence in the application fields where high multiplexity is required, the response time is still slow so that there has been a need for improving the elements.
In such present status, a display mode which has recently been noted is the one proposed by N. A. Clark and S. T. Lagerwall, i.e. a display mode utilizing a light switching phenomenon of ferroelectric liquid crystals (see Applied Physics, Letters, Vol. 36, p. 899 (1980)). The presence of ferroelectric liquid crystals has been stated by R. B. Meyer for the first time (see Journal de Physique, vol. 36, p. 69 (1975)), and from the viewpoint of classification of liquid crystals, the ferroelectric liquid crystals belong to chiral smectic C phase, chiral smectic I phase, chiral smectic F phase, chiral smectic G phase, chiral smectic H phase, chiral smectic J phase and chiral smectic K phase (hereinafter abbreviated to SC* phase, SI* phase, SF* phase, SG* phase, SH* phase, SJ* phase and SK* phase, respectively).
When the light switching effect of ferroelectric liquid crystals is applied to display elements, there are two superior specific features as compared with TN display mode. The first specific feature is that the response is made at a very high rate and the response time is 1/100 or less that of TN display mode elements. The second specific feature is that there is a memory effect, which makes multiplex drive easy coupled with the above-mentioned high-speed properties.
In order for display elements using ferroelectric liquid crystals to have memory properties, two methods are considered. One of these is a method proposed by N. A. Clark et al wherein memory properties are developed by reducing the cell thickness (d) down to a thickness of the helical pitch (p) or less to thereby undo the helix (Applied Physics, Letters, vol. 36, p. 899 (1980)) and the other is a method found by Le Piesant wherein memory properties are developed by utilizing AC stabilizing effect (Paris Liquid Crystal Conference, p. 217 (1984)). The word AC means alternate current hereinafter.
Most current ferroelectric liquid crystal materials have short helical pitches (1 to 3 .mu.m); hence in order to develop memory properties by reduction in the thickness of the cell, proposed by N. A. Clark et al, it is necessary to retain the cell thickness at about 1 to 3 .mu.m, but from the viewpoint of the current cell preparation technique, there is a problem that the retention is difficult in the aspects of cost and yield. On the other hand, the method found by Le Piesant wherein memory properties are developed utilizing AC stabilizing effect is effective only for ferroelectric liquid crystal materials having a negative dielectric anisotropy (.DELTA..epsilon.), but even in the case of thick cells (5 to 7 .mu.m), it is possible to develop memory properties; thus the current cell preparation technique is utilizable and hence the method is very practical.
The AC stabilizing effect is due to a mode utilizing the following fact:
Spontaneous polarization (Ps) results from an impressed electric field in the case where low frequency is applied to ferroelectric liquid crystals, whereas spontaneous polarization does not follow in the case of high frequency; as a result, normal dielectric anisotropy becomes effective and hence if the dielectric anisotropy value is negative (.DELTA..epsilon.&lt;0) liquid crystal molecules are compelled to be in a parallel state to the substrate. Thus, memory properties are developed even in the case of a thick cell. A matrix display utilizing this AC stabilizing effect has been actually reported by Jeary in 1985 for the first time (SID'85, Digest, p. 128 (1985)), but thereafter almost no report has been issued. The main reason for so few reported examples is that there are so few ferroelectric liquid crystal materials having a negative dielectric anisotropy value. Further, according to Jeary's report, a voltage of about 40 V is required for developing memory properties by utilizing AC stabilizing effect, but when usual IC drive voltage range is taken into account, it is desired that AC stabilizing effect be developed at a far lower voltage (25 V or less). As to AC stabilizing effect, the more the negative dielectric anisotropy value, the lower the voltage of the effect developed; hence appearance of ferroelectric liquid crystal materials having a negative larger dielectric anisotropy value has been earnestly desired. Further, the response time of ferroelectric liquid crystal materials reported by Jeary et al is several msecs, that is, the response time is still slow in the aspect of practical use; hence appearance of ferroelectric liquid crystals having a negative dielectric anisotropy value and also high-speed response properties has been desired.